The use of aluminum components in motor vehicles continues to expand due to the relatively good strength-to-weight ratio of this material. However, the expanded application of components made from this material is being hampered because of its limited temperature formability. One increasingly popular method of producing components from aluminum is superplastic forming in which certain materials, including particularly aluminum, are heated (under controlled temperature) and stretched slowly (under a controlled strain rate) to achieve dimensions that are well beyond their normal limitations. Superplastic forming offers a variety of advantages over conventional stamping techniques. Some of these advantages include increased forming strains, zero springback, and very low tooling costs. These alloys can be formed with relatively low forces and they permit a high level of detail in the design of the formed part.
Superplastic forming can result in very deep components which would rupture during the formation process by using conventional methods. The large degree of plastic strain that can be achieved with this process (>200%) makes it possible to form complex parts that cannot be shaped with conventional stamping techniques. As a result, the components produced by superplastic forming processes can embody relatively complex and highly integrated configurations. These components are not only lightweight but also exhibit a high degree of integrity, eliminating not only the number of parts and connectors, but also reducing the number of assembly operations because of the complexities that can be achieved.
Typical superplastic forming takes place in a simple one-sided, single action tool. The blank is clamped in a heated die and then blow formed with gas pressure into a female die. The part detail is captured within a single die rather than a matched pair and therefore tooling is significantly less expensive than that of conventional stamping. Furthermore, the low forces needed to form the material at these elevated temperatures allows for the use of cast iron dies instead of the harder to work and more expensive tool steel.
While superplastic forming may be a viable manufacturing option for some parts, there are limitations in the economic feasibility of this technique. Superplastic response in metals is inherently coupled with the rate of deformation and there exists only a narrow range of strain rates, typically slow strain rates, in which these materials display superplastic response. This results in a relatively slow cycle time which often leaves superplastic forming as a cost-prohibitive option for parts having volumes greater than 1000 parts per year.
Another problem related to SPF stems from the inability to draw material into the die cavity. Although the superplastic material utilized in SPF can undergo substantial deformation, its formability is limited to the amount of material in the die. After the die faces are clamped and sealed, additional material cannot be drawn into the die. This may result in tears or inconsistent wall thickness in the part being formed. To overcome this, U.S. Pat. No. 5,974,847 introduces pre-forming the material around a punch before sealing the dies and completing the forming process by gas pressure injection. This approach reduces the amount of superplastic forming that takes place thereby reducing the cycle time and potentially allowing greater design freedom due to the additional material drawn into the die during the pre-forming step. While the method of this patent teaches pre-forming the material before the gas is injected, the method does not restrain the material entering the die during the pre-forming step. Without a restraining force on the material, such as blankholder force, the material will wrinkle around the punch in all but the simplest of formings. Wrinkling of the material during pre-forming will result in either the inability to complete the part during subsequent gas pressure forming or, at best, a low quality finished part.
In response to the need to reduce the problem of excessive wrinkling of the material during the pre-forming step, U.S. Pat. No. 6,581,428 introduced a method and apparatus which controls the amount of material flow during the forming process. Specifically, this patent teaches control of the amount of material being drawn into the die cavity during a pre-forming process so as to avoid wrinkling of the material.
While the method and apparatus of U.S. Pat. No. 6,581,428 improves the resulting product by reducing the number of wrinkles there is yet room for other advancements in the technology of superplastic forming. The present invention provides such advancement by allowing for significantly faster forming times, improved material utilization, more uniform thinning and the capability of using lower cost aluminum sheet.